A blood sugar level sensor is used for measuring a blood sugar level once in several days to several times in a day, and controlling the measured values in a diabetic patient or a diabetic suspect. As a measurement strip for a blood sugar level sensor, a strip including a base substrate such as a plastic film having a patterned gold thin film is used (see PTD 1). The reason is that gold has low resistance value and high chemical stability that are directly linked to reliability as a sensor.
On the other hand, although there have been a lot of diabetic patients around Europe and America, recently the number of the patients are on upward trend in the whole world such as in Japan, China, and India. Thus, a low cost strip for a blood sugar level sensor has become required. Accordingly, a low cost electrode material without using gold has become required.
As a low cost strip for a blood sugar level sensor, a strip is disclosed which is obtained by depositing a nickel layer with the use of sputtering as one of vacuum deposition methods, and further depositing a carbon layer on the nickel layer to produce an electrode film, and then pattern processing the resultant electrode film (see PTD 2). An electrode material is required to have chemical stability against an electron mediator (a compound mediating electron transfer), such as potassium ferricyanide, having an oxidative effect, a buffer showing acidity or alkalinity, and the like contained in a reagent layer of a strip. The reason for providing a carbon layer on nickel is solely to compensate for low chemical stability of nickel with a carbon layer. The nickel layer is protected to some extent from an acid or an alkali, since the nickel layer is covered over with the carbon layer. However, nickel comes in contact with the reagent layer at a lateral face of the patterned electrode, and this can lead to elution of nickel. Accordingly, an abnormal sensor signal can be observed, or the electrode can be exfoliated.
Further, as another low cost strip for a blood sugar level sensor, a strip is disclosed in which a carbon layer is formed on a substrate, and a metal, such as silver or aluminum, is laminated by vacuum deposition at a region excluding a portion where the carbon layer comes in contact with a chemical reaction portion (see PTD 3). The above technique has been employed because when a metal, such as silver, layer is laminated in advance, and then a carbon layer is laminated on the metal layer, the metal layer is oxidized to increase contact resistance with the carbon layer. Besides, in the same cited literature, the carbon layer is formed by printing ink containing a carbon pigment and an organic binder; however, as a result of a combined use of the printing and a subsequent deposition method of a metal layer via vacuum vapor-deposition method, industrial productivity is significantly decreased, and the carbon layer formed by the printing almost inevitably has a thickness of 0.3 μm to 30 μm, and thus it is not preferred. In the case of the printing, precise control of the film thickness is difficult, and thus there is a problem that a difference in blood sugar level of each measurement with a blood sugar level sensor becomes wider. A vacuum deposition method, such as a sputtering method, is more suitable in that a precise control of film thickness is possible, so that a measurement difference of a blood sugar level can be minimized.